In the fabrication of circuit boards, electronic components are often surface mounted to a bare board via process known as xe2x80x9creflow soldering.xe2x80x9d In a typical reflow soldering process, a pattern of solder paste is deposited onto the circuit board, and the leads of one or more electronic components are inserted into the deposited solder paste. The circuit board is then passed through an oven where the solder paste is reflowed (i.e., melted) in heating zones and then cooled in a cooling zone to electrically and mechanically couple the leads of the electronic component to the circuit board. The term, xe2x80x9ccircuit board,xe2x80x9d as used herein, includes any type of substrate assembly of electronic components.
Solder paste typically includes not only solder, but also flux to promote solder wetting and to provide good solder joints. Other additives can also be included. After the solder paste is deposited on the circuit board, the circuit board is passed on a conveyor through a plurality of heating zones of a reflow soldering machine. As the solder paste melts, volatile organic compounds (referred to as xe2x80x9cVOC""sxe2x80x9d) in the flux and other additives are vaporized and tend to condense in the reflow machine. In many of the reflow furnaces, soldering is now performed in an inert atmosphere using primarily nitrogen to reduce oxidation on solder surfaces.
The heating zones are separated into a number of different zones, including preheat zones, soak zones and spike zones. In the preheat zones and the soak zones the products are heated and the volatile components in the flux vaporize in the surrounding gas atmosphere. The spike zones are hotter than the preheat and soak zones, and it is in the spike zones that the solder melts. A reflow soldering machine may have many heating zones, and these heating zones can be varied depending on the products to be soldered. Different products require different heat profiles, and a soldering machine should be flexible so that, for example, a machine with ten heating zones may have one preheat zone followed by seven soak zones and two spike zones for one type of circuit board, and for a different type of board may have three preheat zones, six soak zones and one spike zone. One or more cooling zones follow the heating zones; in the cooling zones, the solder solidifies on the solder areas of the board.
If volatilized compounds pass from the heating zones into the cooling zones, those compounds typically will condense in the cooling zones; and the condensate can impair cooling functions and may present processing problems. The most common problem occurs with no-clean, enhanced-print-performance solder pastes. These pastes utilize viscosity modifiers to achieve superior printing performance. Problems arise when the viscosity-modification components condense in the cooling zones. By nature, these residues are a viscous liquid and can build up and drip onto the soldered product from the cooling zone surfaces, such as heat exchangers, etc.
Known methods for removing these volatilized organic compounds typically employ heat exchangers that cool a hot gas stream removed from a process chamber and thereby condense organic compounds in the gas stream on a surface of the heat exchanger. The condensate can then be removed with a condensate filter before the gas stream is returned to the process chamber.
In accordance with methods and apparatus disclosed, herein, volatilized organic compounds and other contaminants (e.g., particulates) released in a soldering operation can be removed using a two-stage filtration system including a first stage that traps larger particles in a vapor stream and a second stage including a packed bed of wettable media that traps smaller particles and serves as a nucleation site for condensed organic compounds in the vapor stream.
The two-stage filtration system is particularly useful when coupled with a reflow oven wherein a circuit board is passed through a heated reflow chamber to reflow (i.e., melt) solder paste on the circuit board, thereby volatilizing organic compounds from the flux or other additives in the solder paste. At least a portion of the volatilized organic compounds pass from the reflow chamber through a first stage and then to a second stage of the filtration system.
The first stage is designed to trap larger particles in the vapor stream from the reflow chamber. The second stage includes a packed bed of wettable media designed to trap smaller particles and to serve as a nucleation site for volatilized organic compounds.
In one embodiment, the first stage includes a mesh strainer. The mesh strainer or other filtering element can be employed as a centrifugal self-cleaning element by, for example, coupling the filtering element to a rotary motor that spins the filtering element to centrifugally expel the filtered contaminants from its surface.
Embodiments of the invention can offer some or all of the following advantages. First, the filtration system can yield high collection efficiency, thereby reducing excessive build-up of contaminants within the oven and consequent potential for damage to oven components and/or to the circuit board assemblies passing therethrough if, for example, the condensed flux was to drop onto the assemblies. Second, the filtration system can effectively remove particulate and volatilized contaminants without employing an active-cooling mechanism, such as a heat exchanger; consequently, the filtration system can conserve energy relative to known systems and can minimize facilities requirements (e.g., exhaust, chilled water, compressed gas, etc.). Third, a centrifugal self-cleaning element can be used in the first stage of the filtration system to easily clean the first stage without any additional mechanical or human intervention and with little disruption to the filtration process. Fourth, unlike systems relying solely upon condensation, systems described herein also remove solid particulates entrained in the vapor stream. Fifth, temperatures can remain high in the system because active cooling is not needed, thereby allowing collected contaminants to retain a low viscosity and to drip into easily disposable drain jars. Sixth, the overall simplicity and effectiveness of the system reduces maintenance costs and lost production due to machine downtime.